1. Field of the Invention
The present invention relates generally to a method and apparatus for communication, and more particularly, to a method for the estimation and correction of frequency offset between the local oscillator of a receiver and the carrier frequency of a received signal, and to a radio system having means for frequency offset estimation and correction.
2. Description of the Related Art
A conventional wireless radio system used for telephony (xe2x80x9ccellular systemxe2x80x9d) consists of three basic elementsxe2x80x94namely, mobile units, cell sites, and a Mobile Switching Center (xe2x80x9cMSCxe2x80x9d) In a basic cellular system, a geographic service area, such as a city, is subdivided into a plurality of smaller radio coverage areas, or xe2x80x9ccellsxe2x80x9d. A mobile unit communicates by RF signals to the cell site within its radio coverage area. The cell site""s base station converts these radio signals for transfer to the MSC via wire (land line) or wireless (microwave) communication links. The MSC routes the call to another mobile unit in the system or the appropriate landline facility. These three elements are integrated to form a ubiquitous coverage radio system that can connect to the public switched telephone network (PSTN).
A mobile unit contains a radio transceiver, a user interface portion, and an antenna assembly, in one physical package. The radio transceiver converts audio to a radio (RF) signal and converts received RF signals into audio. The user interface portion includes the display and keypad which allow the subscriber to communicate commands to the transceiver. The antenna assembly couples RF energy between the electronics within the mobile unit and the xe2x80x9cchannelxe2x80x9d, which is the outside air, for transmission and reception. Each mobile unit has a Mobile Identification Number (MIN) stored in an internal memory referred to as a Number Assignment Module (NAM).
A cell site links the mobile unit and the cellular system switching center, and contains a base station, transmission tower, and antenna assembly. The base station converts the radio signals to electrical signals for transfer to and from a switching center.
Digital cellular systems and systems combining analog and digital communication techniques are currently more popular than purely analog systems. Presently, there are three basic types of digital cellular technology: Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). Digital cellular systems currently fall within these three categories and many use a combination of these technologies along with analog techniques. There are also variations in the way radio technologies allow duplex operation, called Frequency Division Duplex (FDD) and Time Division Duplex (TDD).
In order to receive a transmitted digital signal, coherent or non-coherent detection is generally used to extract encoded voice or data contained in the transmitted signal. In a typical coherent detector, for example, the modulated waveform is fed to a mixer, wherein the modulated wave is mixed (i.e., multiplied) with a xe2x80x9clocal oscillatorxe2x80x9d signal having a frequency that is matched to the frequency of the modulated signal.
In time division multiplexed digital communication systems such as the North American TDMA cellular telephone system, information is transmitted as symbols encoded in the phase of the transmitted signal with respect to its carrier. Proper extraction of the symbols necessitates that the local oscillator frequency used to demodulate the received signal is identical to the carrier frequency of the received signal. As is well known in the art, a frequency difference between the carrier of a modulated signal and the local oscillator used to extract the modulated information causes the apparent phase relationship to xe2x80x9crotatexe2x80x9d undesirably.
Certain transmission protocols have the tendency to reduce this effect. For instance, in differential quadrature phaseshift keying (xe2x80x9cDQPSKxe2x80x9d), the encoded information is contained not in the absolute phase of the modulated signal, but in the difference between the phase of a given symbol and the phase of the previous symbol. In an ideal channel, a frequency offset between the local oscillator of the receiver and the carrier frequency of the transmitted signal does not present a significant problem as long as the symbol frequency is much larger than the frequency offset.
The cellular channel is not ideal, however, and is subject to various types of distortion such as delay spread due to multipath fading, Doppler effect, and the like. A process such as adaptive equalization, which involves adaptive channel distortion characterization, is needed in order to extract symbols accurately from the time-varying channel. To estimate and compensate for channel-induced distortion, cellular systems typically utilize adaptive equalization techniques which predict the channel response based upon the transmission of known data (eg., a so-called pilot signal or training sequence). However, such processes are sensitive to significant uncorrected frequency offsets, which may cause the channel to vary beyond the rate at which the adaptive processes can adapt. Even for DQPSK systems, therefore, accurate frequency offset compensation is necessary.
A conventional approach to frequency tracking is by use of a phase-locked-loop (xe2x80x9cPLLxe2x80x9d). A PLL circuit is typically formed as a phase detector fed by input and feedback signals, a loop filter and a voltage controlled oscillator for producing a sine wave (i.e., the feedback signal). In a PLL, the phase of the received signal, or a frequency-translated version thereof (i.e., an intermediate frequency (IF) signal), is compared with the local phase reference (i.e., the local oscillator), and the average phase difference over time is used to adjust the frequency of the reference. A basic PLL is characterized by a pull-in range Bp. However, as Bp increases, so does the variance of the phase error. AFC (Automatic Frequency Control) units, FLLs (Frequency Locked Loops) or PLL""s with phase and frequency detectors are often used to track such signals. These circuits generally produce an estimate of the average input frequency only, and additionally require an elemental PLL if the phase is to be acquired. Unfortunately, phase-locked-loop systems tend to result in a cellular telephone system can be unacceptable. In addition, in cellular systems based on packetized data transfer, control data is often contained in a single packet, which may be lost before phaselock is achieved. An objectionable amount of dead time may also be encountered during handoff from one cell to another. This is true both for conventional, analog phase-locked-loop systems and for digital equivalents. Moreover, in wireless communications AFC design has been constrained by circuit complexity, and system designs have typically made frequency accuracy constraints somewhat loose to avoid prohibitive costs in complexity or processing requirements.
In addition, with the introduction of more optimal modulation schemes such as QPSK, relatively precise frequency estimates are often needed. Frequency errors may arise, for example, from the transmitter/receiver clock not being perfectly locked due to inaccuracies or drift in the crystal oscillator, as well as from large frequency shifts due to the Doppler effect, such as those occurring from vehicles moving at high speeds in open spaces. Many cellular systems allow only a small amount of time for achieving initial signal acquisition and require a minimum tracking error after initial acquisition. However, typical AFC or PLL circuits are not generally able to lock on or track a received signal with wide frequency shifts over a short period of time with a reasonable degree of accuracy.
The widespread demand for increased functionality and capacity in mobile duplex communications equipment has resulted in a rapid advancement in wireless technology. Over the past ten years, for instance, wireless telephony end-user equipment size, weight, and cost have dropped over 20% per year while advanced techniques in efficient bandwidth utilization have led to a tenfold increase in system capacity. To satisfy the increased demand in system capacity over conventional analog cellular systems, the Telephone Industry Association (TIA) promulgated in the late 1980""s an Interim Standard for time division multiplexed wireless digital telephony, known as IS-54. As is well known in the art, IS-54 (revs. A and B), and the more current Interim Standard for time division multiplexed wireless telephony, IS-136, use Time Division Multiple Access (TDMA) digital technology, which is well known in the art. The reduced size, weight and cost of such consumer-based equipment is largely the result of increased integration density of mobile unit circuitry coupled with the development of low power, low range digital mobile units operating in a multiplexed environment and stationary equipment capable of handling the increased capacity.
Efforts to reduce the cost of cellular equipment have also led to the to the use of software executed by a Digital Signal Processor (xe2x80x9cDSPxe2x80x9d) or other microprocessor installed in cellular end-user equipment to perform signal processing techniques conventionally performed solely by circuitry. For instance, frequency offset estimation and correction are often accomplished in software by an algorithm which determines frequency offset based upon the use of a known data such as a pilot sequence. In some applications, software is used to supplement a conventional PLL circuit to compensate for frequency offsets left uncorrected by the PLL. In other applications, software correction for frequency offset is performed to compensate for driftin a quartz crystal oscillator used in a mobile unit. Such techniques permit the use of lower precision circuit components than otherwise possible and facilitate a reduction in the the cost and size of mobile equipment. Certain digital techniques perform the local-reference adjustment relying solely on data extracted from the carrier in order to estimate the offset. Such techniques are not sufficient in a time-dispersive channel environment, however, because the process for extracting the data depends on adaptive processes, which are themselves relatively intolerant to significant frequency offsets. Such techniques for adjusting the local reference frequency can generally function only after the local oscillator frequency has already been adjusted.
Many conventional frequency offset correction techniques rely upon the premise that frequency offset can be estimated by monitoring the phase difference between successive symbol samples in a received waveform. Since frequency mismatch essentially results in a fairly constant phase offset in a received waveform over an entire burst of symbols, detection of this phase offset would provide a sufficiently accurate estimate of the frequency offset. One digital approach, described in U.S. Pat. No. 4,491,155, performs coherent demodulation of a received TDMA signal by producing a vector sum of reconstructed in-phase and quadrature phase signals. The phases of the incoming signal add constructively only at the optimal sample, and destructively in all other cases. Thus, the reconstruction signal having the greatest magnitude is used determining the phase shift due to frequency offset.
Another digital method, disclosed in U.S. Pat. No. 5,499,273, performs a successive accumulation over substantially an entire burst of sampled in-phase and quadrature phase (I and Q) components of a received signal to determine not only the frequency offset but also the sampling phase error. Magnitudes of received in-phase and quadrature phase sampled signals are determined, summed and distributed to accumulator registers for accumulating sums of the first and second signals for each sample time and substantially over a length of an expected burst. A maximum-minimum determination circuit chooses the sample time having the largest or smallest sum to provide a recovered clock signal. The carrier is recovered, and a downsampler downsamples the received in-phase and quadrature phase signals based on the recovered clock signal. In order to attenuate data-dependent effects sufficiently so that the frequency and phase-dependent effects dominate, however, it is necessary to average the phase relationships over a large number of symbols, which, in most cases, is an entire burst. This can result in a significant delay in achieving a sufficiently accurate frequency offset estimate.
In another approach, described in U.S. Pat. No. 5,588,026; the detected signal is raised to the M-th power to remove a modulation factor from the received signal. The M-th power signals for N symbols are accumulated to derive a phase component of the M-th power signals. The derived phase component is then divided into M to obtain an estimate of the phase shift due to frequency offset which represents a deviation from a true carrier frequency contained in the received signal. The received signal is then multiplied by the obtained estimation value in a form of a complex conjugate to remove the phase shift due to the frequency offset from the received signal. While this method provides for offset correction by use of a simplified software routine, it is effective only when the phase shift due to frequency offset is within a range df xe2x88x92xcfx80/M to xcfx80/M and is thus subject to uncertainty if the phase shift exceeds this range. In other words, since the estimation value of the phase shift is calculated as a value between xe2x88x92xcfx80 and xcfx80, a phase shift larger than xcfx80/M contained in the received signal prior to the M-th power operation cannot be detected by the M-th power operation. To avoid this phase ambiguity, the method requires that the transmitted signal is differentially encoded twice, which requires additional processing and specially designed transmitting equipment which is generally incompatible with industry standards.
Accordingly, there is a need for a sufficiently rapid and simplified method and apparatus for frequency offset estimation and compensation which minimizes processing time and optimally avoids the need for additional circuitry.
The time and processing requirements needed to achieve an accurate frequency offset estimate can be minimized, even in systems that are to be used in time-dispersive channels by effectively adjusting the frequency of a receiver local oscillator in accordance with a xe2x80x9cphase rotationxe2x80x9d observed in the relationship between a plurality of successively received symbols. The present invention is based upon the recognition that certain modulation techniques exhibit statistical characteristics which are detectable despite the presence of severe channel distortion.
In xcfx80/4-shifted DQPSK, for example, the potential differential phase angles between two successively received symbols over an ideal channel will be xc2x145xc2x0 or xc2x1135xc2x0. While channel effects produce results which deviate from ideal conditions, even in the presence of severe channel distortion due, for example, to multipath effects and Doppler effect, there is a statistical concentration of differential phase angle values in the vicinity of +45xc2x0 or xc2x1135xc2x0.
In order to correct for frequency offset according to the present invention, therefore, selected pairs of successively received symbols in a communication system are multiplied to produce a set of vectors each having a phase angle representing the differential phase angle between the successive symbols. The resultant vectors are rotated so that the in-phase and quadrature phase components are positive. In xcfx80/4-shifted DQPSK, the angle of the rotated values, in an ideal channel, should be 45xc2x0. A deviation from 45xc2x0 is observed, which is indicative of xe2x80x9cphase rotationxe2x80x9d due to frequency offset between the local reference and the carrier. This phase rotation can then be used to adjust the local reference frequency. Since this approach eliminates the need to average over long periods, the need for filtering out of data-dependent effects, and the use of known data sequences, the time required to achieve adequate frequency-offset compensation is shorter in many important environments than it is for conventional systems.